A continuing program in laboratory animal nutrition involves studies with the species that are most frequently used as animal models in biomedical research. The objective of the program is to improve the nutritional status of NIH production and research colonies of rats, mice, guinea pigs, rabbits, dogs, cats and various species of nonhuman primates. A series of factorially designed feeding trials are used to ascertain the nutrient requirements of the species of interest and to develop diets with more nearly optimal nutrient concentrations for their growth, reproduction, maintenance or general health status. This program has resulted in the development of open formula natural ingredient diets for all animal species maintained in significant numbers at the NIH except for dogs and cats.